Definition of Toe the Party Line

What does the term "toe the party line" mean? What is the definition of the term "toe the party line"?

If somebody is said to "toe the party line", this means that they are falling in line with the general ideals and principles of the party and voting along those lines.

Example

Joe Smith is a Republican senator from Alabama. Congress is voting on whether or not to send troops to a Mideast country for a military operation.

Democrats are largely opposed to the operation, while Republicans support it. Smith has some reservations about the operation, but he votes in favor of it anyways. Smith has aspirations being a Presidential candidate one day, so he doesn't want to ruffle any feathers in his party by voting against the mission.

By voting in favor of the military operation, Smith is "toeing the party line", despite his own personal reservations about the operation.